1. Field of the Invention
The present invention relates to a color image reading apparatus and, more particularly, to a color image reading apparatus, which can accurately read color image information on the original surface using a simple, monolithic 3-line sensor, and is suitable for, e.g., a color scanner, color facsimile apparatus, and the like, since it can relax required mechanical precision by broadening the allowable range of synchronization errors caused due to the attachment precision of a scanning mirror, parallelness between the original surface and a guide rail that guides a scanning mirror unit, and the like, and corrects any asymmetry among the spacings of a plurality of color light beams, which are color-separated in the sub-scanning direction, on the surface of a light-receiving means.
2. Related Background Art
Conventionally, various color image reading apparatuses have been proposed. In such color image reading apparatus, color image information of, e.g., an original or the like is scanned in units of lines in the sub-scanning direction, and is imaged on the surface of a monolithic 3-line sensor (to be simply referred to as a “3-line sensor” hereinafter) serving as an image reading means (light-receiving means) via an imaging optical system, thus reading the color image information of, e.g., the original or the like using an output signal obtained from the 3-line sensor.
FIG. 1 is a schematic view showing principal part of a conventional color image reading apparatus of this type. Referring to FIG. 1, a light beam reflected by a color image on an original surface 51 illuminated with an illumination means (not shown) is imaged on a surface of a 3-line sensor 59 via an imaging optical system 54, thus reading color image information of, e.g., the original or the like using an output signal obtained from the 3-line sensor 59.
As shown in FIG. 1, when a color image is read by the 3-line sensor 59 using the normal imaging optical system 54 alone, the reading positions on the original surface 51 that can be simultaneously read by three line sensors 59a, 59b, and 59c become three different positions 51a′, 51b′, and 51c′. 
For this reason, three color signal components (R, G, and B) of an arbitrary position on the original surface 51 cannot be simultaneously read, and after these components of the arbitrary position are separately read by the 3-line sensor 59, they must be registered and synthesized.
For this purpose, spacings S1, and S2 between adjacent lines of the 3-line sensor 59 are set to be an integer multiple of a pixel size W2 of each pixel 58, as shown in FIG. 2, and corresponding redundant line memories are prepared. Then, G and R signals (signal components based on G and R color light beams) are delayed with respect to a B signal (a signal component based on a B color light beam), thus relatively easily obtaining synthesized three-color signal components.
However, upon assigning redundant line memories in correspondence with the inter-line distances of the 3-line sensor 59 in the above-mentioned color image reading apparatus, a plurality of lines of expensive line memories must be prepared, resulting in disadvantages in terms of cost, and posing problems, e.g., a complicated arrangement of the overall apparatus and the like. Note that FIG. 2 is an explanatory view of the 3-line sensor 59 shown in FIG. 1.
FIG. 3 is a schematic view showing principal part of a conventional image reading apparatus which color-separates color image information of an original using a beam splitter for color separation into three, i.e., R, G, and B color image signals, and simultaneously reads the three, i.e., R, G, and B color image signals.
Referring to FIG. 3, when a light beam reflected by a color image on the original surface 51 illuminated with an illumination means (not shown) is imaged on the surface of the 3-line sensor 59 by an imaging lens 74, the light beam is color-separated into three light beams (color light beams) corresponding to, e.g., three, i.e., R, G, and B colors, via two beam splitters 78a and 78b for color separation. Then, color images based on the three color light beams are imaged on the surfaces of the line sensors 59a, 59b, and 59c of the 3-line sensor 59. In this way, the color image is scanned in units of lines to read that image in units of color light components.
However, the beam splitters 78a and 78b for color separation in FIG. 3 require a very thin glass plate since three layers of dichroic mirrors are placed parallel to each other, and such structure is very difficult to manufacture.
FIG. 4 is a schematic view (sub-scanning sectional view) showing principal part of a conventional reading apparatus in the sub-scanning direction, which color-separates color image information on an original into three, i.e., R, G, and B color image signals using a reflection linear blazed diffraction grating, and simultaneously reads three, i.e., R, G, and B image signals using a 3-line sensor.
Referring to FIG. 4, a 3-line sensor 89 is used as an image reading means, and a reflection linear blazed diffraction grating 88 serving as a color-separation means is placed in the imaging optical path to be separated from the exit pupil of an imaging lens 84 in the direction of the 3-line sensor 89, so as to attain color separation using reflection and diffraction. Color image information for one line on the original surface 51 is color-separated and imaged on the surface of the 3-line sensor 89, thus reading the color image information.
The reflection linear blazed diffraction grating as the color-separation means in FIG. 4 can be easily manufactured but suffers the following problems. More specifically, in FIG. 4, in order to color-separate a light beam into three, i.e., R, G, and B color light beams (diffracted light beams) using the linear blazed diffraction grating, e.g., to obtain G light rays as 0th-order diffracted light, R light rays as first-order diffracted light, and B light rays as -first-order diffracted light, angles the ±first-order diffracted light components make with the 0th-order diffracted light do not match each other independently of the pitch setups of the linear blazed diffraction grating, and some asymmetry remains unremoved. For this reason, the color light beams have different spacings on the surface of the 3-line sensor 89.
Hence, conventionally, a special sensor which has asymmetric line spacings in the sub-scanning direction, i.e., does not have normal equal line spacings is used as a 3-line sensor, or an optical element for correcting the spacings of the color light beams color-separated by the linear blazed diffraction grating to be equal to each other on the 3-line sensor must be inserted into the optical path between the linear blazed diffraction grating and 3-line sensor.
Generally speaking, the spacings S1, and S2 between the adjacent lines of the 3-line sensor preferably assume equal values in terms of an easy semiconductor process.
Other problems of the conventional color image reading apparatus having the aforementioned color-separation optical system will be explained below with the aid of FIG. 5. Note that the same reference numerals in FIG. 5 denote the same parts as those in FIG. 3.
Referring to FIG. 5, when color image information at a point A on the original surface 51 is color-separated into three color light beams using the two beam splitters 78a and 78b for color separation, and these color light beams are focused on the surfaces of the corresponding line sensors 59a, 59b, and 59c, color image information from a point B, which is in the neighborhood of the point A, e.g., a light beam having G (green) image information is reflected by the beam splitters 78a and 78b at the same time, and is focused on the R line sensor 59a which does not correspond to that G (green) image information. Such problem is generally called crosstalk of color information in the sub-scanning direction, and upon reading color image information, such crosstalk is one of causes that produce image disturbance.
The crosstalk in the sub-scanning direction occurs not only in the color image reading apparatus using the beam splitters for color separation, as shown in FIG. 5, but also similarly in a color image reading apparatus using the linear blazed diffraction grating as the color-separation means, as shown in FIG. 4.
Conventionally, in order to prevent crosstalk of color information in the sub-scanning direction, i.e., to intercept a light beam coming from the point B in the vicinity of the point A on the original surface 51, a small slit 56 is placed in the vicinity of the surface of the original 51.
However, the small slit 56 which has the same main scanning length as that of the original surface 51, and has a very small sub-scanning width must be accurately attached in the vicinity of the original surface 51, and at the same time, synchronization errors must be accurately suppressed so as to prevent a light beam from being eclipsed by the slit. Such structure is mechanically very difficult to obtain.